In recent years, in order to realize low fuel consumption of an automobile, it is strongly desired to make the automobile light in weight, and a high-strength steel sheet is required from such a viewpoint. From another viewpoint of improving safety performance against a collision also, for a structural member for automotive use such as a rocker, for example, ultrahigh strengthening as high as 1,100 MPa or above is required. In such an ultrahigh-strength steel sheet, a problem of hydrogen embrittlement occurs by infiltration of hydrogen generated by corrosion reaction under an environment including water, hydrogen sulfide and the like.
Also, in a galvanized steel sheet performed with hot-dip galvanizing or alloyed hot-dip galvanizing because of the rust prevention requirements, hydrogen embrittlement occurs by performance of hot-dip galvanizing or alloyed hot-dip galvanizing because of hydrogen occluded in pickling after hot rolling. In a galvanized steel sheet also, the hydrogen embrittlement problem becomes conspicuous in an ultrahigh-strength area of 1,100 MPa or above tensile strength in particular.
Further, although bending workability is required for a steel sheet for automotive use, as the steel sheet is high strengthened, bending workability tends to be deteriorated, therefore a technology for securing excellent bending workability even in an ultrahigh-strength area of 1,100 MPa or above is required.
With regard to the documents related to the ultrahigh-strength steel sheet, the Japanese Patent No. 3,254,108, for example, discloses a 1,180 MPa or above ultrahigh-strength steel sheet with improved hydrogen embrittlement resistance containing compositions such as Ca, Cr, Ni and Cu. Also, “Effect of old γ grains refinement on delayed fracture resistance of 1,400 MPa class high-strength steel” by Yuji Kimura and 4 others, CAMP-ISIJ, Vol. 14 (2001)-1310 discloses that control (refinement) of the grain diameter of old austenite grains is effective in improving delayed fracture resistance of 1,400 MPa class steel.
Also, the Japanese Unexamined Patent Application Publication No. 2005-171321 discloses a 980 MPa or above high-strength steel sheet whose formability and bending workability are improved by optimizing the grain diameter of ferrite and the fraction and hardness of a low temperature transformation formation phase, although 1,100 MPa or above ultrahigh-strength level is not a direct object of the patent.
In both the documents, either one of hydrogen embrittlement resistance and bending workability is watched, but an ultrahigh-strength steel sheet in which both characteristics of hydrogen embrittlement resistance and bending workability are improved is not disclosed.